Powder metallurgy



Patented Jan. 14, 19 36 2.0 1.532 rowers rm'rnmu'nor f Charles fiardv.Pelham, N. Y. ass s' or to a; ll slcal Copany, .New York, N.- Y acorporation of ware No Drawing. Application April a, 193% A Serial No.719,935

. 2 Claims. This invention relates to the manufacture of metallicarticles from finely" divided metals or metal powders and has for itsobject improvements in the methods of producing the same. 5 Metalpowders, such as those used in the formation of metallic objects, arefrequently produced in a wet state; .thus for example, copperpowders areelectrolytically deposited from, a solution of copper salts. Metallicpowders may also be precipitated from solutions through the use of asuitable reducing agent. In many instances, powders precipitated fromsolution have a dendritic or tree-like structure when firstprecipitated. Frequently such metallic powders are soft and amorphous'incharacter. I

. According to the heretofore customary practice such powders are washedto free them of soluble impurities; Usually the final washing step isconducted with alcohol or some other volatile liquid which tends topreserve the metal powders against oxidation. The powders are then driedat a fairly high temperature and in the dry condition they arecompressed and sintered to form a coherent mass of suitable density and2s. shape.

In the case of metal powders produced by wet methods, the dryingtreatment is ordinarily responsible for a marked change in the-:physicalstructure of the particles. Even at moderate temperatures the dendriticstructureis lost to a large extent and the hardness of the particles isconsiderably increased. In thejcase of copper powder, this change isparticularly marked. The powders which are originally soft, of lowdensity and tree-like in their structure, become-more compact and losetheir irregular configuration, even when subjected to a temperature of300 centigrade fora period of 60 minutes. The dried product is granularas compared with the amor- 49 phous character of the wet powder.

Objects made from dry metal. powders have satisfactory characteristicsif a sufficiently high compressive force is applied in theirmanufacture. In some instances. however, a force of 50 tons per squareinch is necessary to produce a coherent non-porous mass, and thenecessity of such a high degree of compressive force has preeluded-theuse of the process of powder metal- 1 lurgy in the case of comparativelylargeobjects. This will be apparent whenit is realized that to form anobject with a cross sectional area of two square feet may; require theapplication of a totalforce of approximately 14,000 tons. The dimcultyof manufacture is further aggravated by the fact that this pressureshould be uniformly applied overthe entire cross section of the body.Even with the most modern of hydraulic presses a uniform application ofa force of this magnitude is difiicult, if not impossible. In theheretofore customary art of powder metallurgy the use of substantiallydry powders has been considered necessary for several reasons first,because it was believed that the presence of a substantial quantityof,liquid interfered in the production of a satisfactory metallicarticle;

second, because the difllculties of handling wet powder in a mold werethought to be considerable,

, and, third, in the case of a powder precipitated from solutions theamorphous or dendritic structure was considered to be less satisfactorythan 15 the granular form which pertains in the case of dried powders. IAs a result of my investigations! have discovered that it is possible toproduce a very satis factory metallic object by employing metal powgoders in a .wet state. Thus. for example, copper powders produced byelectrolytic deposition from solution maybe washed and compressed in amold while they still contain a considerable quantity of entrainedmoisture. After si-ntering, an object produced in this manner is .in noway inferior to one produced from dried copper powders. Moreover, thecompressive force which is necessary in order to produce an object of agiven density is but a small fraction of that required when dry powdersare employed.

According to a, presently preferred practice of my invention wet metalpowders are washed with water to remove entrained electrolyte ordissolved impurities. The washed powder is then placed in a centrifugeand a portion of the entrained water is thus removed. The powder whilestill retaining a considerable percentage of moisture is then placed ina suitable mold and compressed under a pressure of approximately 2.5tons per square inch, and a coherent substantially nonporous mass isthus produced. The mass is subjected to heat treatment at a temperaturebelow its melting. point and preferably in a reducing atmosphere inorder to preclude oxidation of its 5 surface. The resulting object iscompact. and

possessed of great tensile and compressive strength. In the event that aporous mass is desired the pressure may be considerably reduced.

objects produced by the compression of wet pow- 1 ders may be wettedwith the requisite amount of a non-viscous liquid of non-oxidizingcharacter, such as water, alcohol, acetone, or benzine, compressed at arelatively low pressure and sintered.

compressed in the wet state. with regard to metal powders which, havebeen The optimum amount of wetting agent will vary with the character ofthe powders employed and the required density of the finished object. In

some instances the amount of liquid which will no be entrained in anuncompacted mass of the powder may be desirable. In other cases a lesseramount of wetting agent produces satisfactory results. However, I havediscovered that more power and a higher degree of compressive force arerequired when powders produced by dry methods are wetted and subjectedto'compression, than when powders precipitated from solution are This isalso true precipitated from solution, subjected to a milddrying-treatment and subsequently wetted and compressed. Such practicealso required greater compressive stress than in the case of freshlyprecipitated wet powders which are compressed without a dryingtreatment. I attribute this phenomenon tov the fact that the softness offreshly precipitated powders enables compressionto be accomplished withless friction between particles and between the particles and the wallof the die or mold.

' According to another practice of my invention I take two or moredissimilar metal powders, at

least one ofwhich is in a wet state, and mix them to secure thoroughincorporation. The mass,

while still wet is subjected to a compression and heat treatmentoperation in "the conventional manner. This method of operation isadvantageous when it is desired to produce an object into by drymethods, are to be incorporated. It sometimes happens that the amount ofliquid naturally entrained in a 'powder produced by wet methods is morethan is required to obtain minimum pressure in the compressionoperation. Dry powders may thenbe added to reduce the average liquidcontent of the mass.

When two or more metal powders of's'ubstantially diiferent specificgravities are mixed, a centrifuging operation to remove excess water isundesirable, since a certain amount of segregation results. when suchtendency for segregation emsts I have determined that it is preferableto remove excess liquid by means of a filter of either. the vacuum orpressure type.

It is frequently desirable to produce metallic objects from metalpowders the surfaces of which have been plated witha dissimilar metal.By

greater proportion of the object. The use of plated metal powders isalsodesirable when it is necessary to combine two or more'metals inproportions in'wliich' they will not alloy with one. another. Since theplating of metal powders is usually accomplished by wet methods-eitherby accuses electrolytic deposition or by chemical replacement, myprocess is particularly applicable to the manufacture of metallicobjects from plated metal powders. For example, iron powder may becoated with copper by rapidly agitatinga. mix- 5 ture of iron powder andcopper sulphate solution. By controlling the respective amounts andconcentration of iron powder and copper sulphate I am able to produce auniform copper plating of the desired thickness on the iron particles;The resulting powder is then washed to remove iron "alts and excesscopper salts and while still wet is compressed to the desired degree ofdensity, and heat treated until the copper binds the iron particles.together. In the event that other ingredients are desired these may beadded in dry or wet powder form to the wet plated powder.

According to another aspect of my invention I may combine two or moremetallic elements by first preparing an alloy in powder form by meth-.iods of wetprecipitation; this powder being subsecarbon disulphide mayalso be used, but since the temperatures generated in the compression ofthe powders may be high, the use of inflammable liquids such as alcohol,or benzine introduces a fire hazard. It should also benoted that inmost instances the use of water as a wetting agent has resulted in amore satisfactory product or in a 40 lower compressive force. Thermovalof soluble impurities from the metal powders, by washing out entrainedelectrolyte etc., is usually desirable, but is not essential to thepractice of my invention, since practice has demonstrated that en- 45trained electrolyte may be almost completely squeezed out in the mold. 1

The wet powders do not present inordinate handling difficulties in thecompression step. In factthe wet character of the powder actuallyimproves the operation in that it prevents dusting. This is especiallyadvantageous where such dust may be carried into the lungs of theoperator.

The amount of liquid present with the powder, may be varied to. aconsiderable degree. Due to the fact that the mold must be provided witha means for egress of theliquidduringcompressionit is undesirable toemploy quantities in excess of that required to obtain a minimumcompressive 60 force for the degree of density which is desired. I havefound that in most instances the amount of water which isretained in apowder that has been subjected to a mildcentrifuging or filtrationoperation is suflicient for this purpose. The 65 quantity ofthe finishedobject,however, is not affected by the use of liquid in excess of thatrequired for minimum compressive force. In the case of copper powders ofa fineness of approximately 300 mesh which have been produced by 7electrolytic processes, I have found that a moisture content ofapproximately 15% is desirable. It will be apparent that the amount ofwetting agent present will be dependent upon the nature oil thepcwdersemployed, that is, their composi- 16..

tion, fineness and physical structure, and also upon the characteristicswhich are desired in the finished object.

The advantages of my invention may be enumerated as follows:

1. In the case of powders which have been prepared by wet methods thedrying step is eliminated, thus saving time, labor and reagents, such asalcohol, which are employed to prevent the oxidation of the powderduring the drying operation.

2. There is a great saving in power, since when wet powders are employedthe force necessary to produce a mass of predetermined density is muchless than whendry powders are employed.

3. Less compressive force need be applied in the case of wet powders,making it possible to produce much larger articles without encounteringthe difliculties which arise from an unequal dis-'- tribution ofcompressive stress.

4. Improved operating conditions.

My methods are applicable to mixtures of two or more metal powders. Ihave found that powders of iron, copper, zinc, and other metals'may besatisfactorily employed either alone'or mixed with one or more othermetals. In certain instances it is also desirable to mix non-metallicpowders, such as porcelain dust, or other materials of highdielectricstrength with wet metal powders in order to produce structuresof high electrical resistance. As has been noted, the process is alsoapplicable to plated powders. In

' many instances, it may be desirable to mix powders which have beenprepared by dry methaman skilled in the art.

- I claim:

1. In a process for making a metallic object in which metal powders thathave been platedwith metal in an aqueous'solution' are employed, the 15improvement which comprises compressing the plated powders into acoherent mass while said plated powders are still wet with water andbefore the original soft amorphous character of the plating has beenaltered substantially by heating, 20 thereby reducing the compressiveforce necessary to form a coherent mass of predetermined density. v

2. Ina process for making a metallic object in which metal powdersdeposited from an aqueous 25 v solution are employed, the improvementwhich comprises "compressing said powders into a coherent mass while thepowders are still wet with water and before the original soft amorphouscharacter of 'the powder has been altered sub 30 stantially-by heating,thereby reducing the compressive force necessary to form a coherent massof predetermined density.

CHARLES HARDY.

